Melbourne-based Zeljka Pokrajcic is a metallurgical engineer on a mission to change the way companies plan and execute mineral processing. In 2010 she completed an award-winning PhD thesis on the subject of comminution, which she puts to good use as a founding director of the Coalition for Eco-Efficient Comminution (CEEC), a non-profit dedicated to optimizing the processing efficiency of mines everywhere. By day she works at Advisian, a global management consulting group that is part of WorleyParsons, where she conducts optimization and trade-off studies for the minerals industry.
CIM: How did you get interested in mining and comminution?
Pokrajcic: In high school I had an ability in maths and sciences, and I knew I wanted to do an engineering discipline, but I wasn’t sure which one. A friend was doing chemical engineering and mentioned the possibility of a metallurgical degree. My first question was, what is it, and how do you spell that? I did some research and thought it looked very interesting. There was lots of travel involved, and lots of big exciting machinery. So I signed up. In the first year, I was one of four females in a group of 40.
CIM: You came to Canada to work as an engineering undergrad. What did you do?
Pokrajcic: I have a real soft spot for Canada; it was the launching pad for my career and my foray into processing. I managed to get a job in Canada when I took a year off in between my third and fourth year in university, at Lakefield Research just outside of Toronto, which is now SGS Lakefield Research. I worked about six months there, and found another job outside Thunder Bay at Winston Lake with Inmet Mining, which has since closed. I had a good year in Canada working in mines and research facilities, which really cemented my commitment to the industry.
CIM: You write in your thesis that the long-term viability of the industry will ultimately depend on improving current “generic” approaches to comminution. What do you mean by that?
Pokrajcic: If you consider a crusher, which is the first step in size reduction, essentially the design and the way that this piece of equipment operates hasn’t changed that much. Also, the design of new comminution circuits follows one or two standard designs, which have been around for about 50 years, featuring the same equipment in the same format. Meanwhile, we’re getting into deeper resources, which are lower grade, and we have to use more energy to extract the valuable minerals. So we’ve got to be smarter with how we design and run these circuits.
CIM: How can it be done smarter? You have suggested that a mining operation needs to know a lot about the ore they are digging up, instead of just setting up a generic circuit.
Pokrajcic: Yes, that’s a key point. We need to tailor the design of the comminution process to suit the ore properties. We now have methods and tools available to us to tell us what is going on in the ore body and what the different properties are. If the ore body has a tendency for the valuable minerals to congregate in the fine particles, let’s put in a screening stage. And if the ore body can be differentiated by colour, grade or fluorescences, let’s consider an ore-sorting stage. It’s these kinds of things that we should be exploring.
CIM: It seems logical to avoid putting useless waste rock into a comminution circuit from the outset.
Pokrajcic: Yes, that’s called pre-concentration, which means exploiting the inherent features of the ore body to reject waste material. By removing waste from further processing you will end up with a smaller circuit, so you have a smaller capital cost and a lower operating cost.
CIM: What does pre-concentration look like in practice?
Pokrajcic: Depending on the ore properties, it could be as simple as putting in a screen or separation device based on size, so as soon as the material comes out of a size-reduction process, it goes over a screen and you can separate one size fraction from another. Typically, one size fraction is higher grade than another, so you reject some and put the rest into the processing circuit. Or perhaps the ore body exhibits a difference in specific gravity, containing minerals that are either high or low in specific gravity. In this case a heavy media separation can be employed. This mode of separation has been around a long time and it’s well practised.
CIM: As far as energy savings, you note that comminution is the most energy-intensive part of the mineral processing stage.
Pokrajcic: It typically uses up to 40 per cent of the total energy used in a mineral processing circuit. But it’s possible to improve energy efficiency by up to 30 per cent on existing circuits by finessing and tweaking their operation. You don’t have to put in a whole new mill, automation system or sorting system. There are things that you can do that are available to operators now, like optimizing mill operation and performance, including liner and grate design, charge composition and particularly feed size.
CIM: What is CEEC doing to make smarter comminution a reality?
Pokrajcic: One of the projects we’re working on with the Canada Mining Innovation Council (CMIC) is a program called the CEEC Energy Curve, which is a benchmarking tool that allows operators see where they are compared to other sites in terms of comminution energy efficiency, so you can ask, “What can I do to improve my operation?” The program addresses some of the low-hanging fruit. The first stage of realizing the opportunity is to benchmark, and that’s what the CEEC Energy Curve is all about.
CIM: Can you cite any examples where operating mines are incorporating the kinds of comminution circuit improvements you are championing?
Pokrajcic: At the Antamina mine in Peru their aim was to increase plant throughput with little or no capital investment. They optimized rock breakage and fragmentation from blasting and reduced run of mine top size and SAG mill feed size. Their throughput increased from 2,750 tonnes per hour to 3,600 tonnes per hour. Another example is Barrick Gold, which in 2010 targeted three operating sites – Cortez, Cowal and North Mara – where they improved SAG mill liner and lifter design, incorporated better crushing practices and reduced mill drive losses. The changes resulted in total energy savings of 61 million kilowatt hours (kWh) per year, worth about $5 million annually.
CIM: What drivers are pushing companies to make efficiency improvements to their comminution circuits?
Pokrajcic: For a lot of these generic circuits that are already installed, operators are often looking at optimizing because they want to increase throughput and reduce costs or because the ore body properties have changed. They’re looking to run their operations faster and cheaper, which means assessing different options for improved efficiency and linking these to ore properties, which wasn’t considered previously. It’s an interesting driver and I’m glad it’s happening.
CIM: What are the barriers to making these kinds of improvements?
Pokrajcic: There’s a really high turnover at sites in terms of people, so a lot of knowledge and experience is lost every couple of years, every time a new group comes through. Also, I think that as an industry – and this is starting to change, but certainly earlier on – it was really tough to convince people to embrace new technologies and ways. There’s a saying in our industry that everybody wants to be second. Nobody wants to be first and risk the possibility of not being successful and losing a lot of money as a result. Mineral processing is a very large, capital-intensive industry, so if you make a mistake it’s going to be costly.